JPH0426232B2 - - Google Patents

Description

[Detailed Description of the Invention] <Field of the Invention> This invention relates to a semiconductor laser,
It particularly relates to lasers having non-absorbing regions adjacent mirror facets.

BACKGROUND OF THE INVENTION A typical semiconductor laser generally consists of a body of material, such as a group compound, an alloy of such compounds, etc., with a thin active layer between layers of opposite conductivity type.
The laser described in U.S. Pat. No. 4,347,486 and the Applied Physics Letters
Physics Letters), published on August 15, 1982,
Limited 2 published by Connolly et al., vol. 41, No. 4, pages 310 to 312
Heavy heterostructure lasers can generate high-power laser beams in single transverse (perpendicular to the plane of the laser) and lateral (direction of light propagation) modes. These lasers have a guiding layer adjacent to the active layer, so that the light generated in the active layer mostly propagates in the adjacent guiding layer, thereby creating a larger facet over which the light can be generated. creating an area. Light emission from one of the mirror facets of such a laser still occurs only from a small portion, typically only a few square microns of the mirror facet of the device. Therefore, the local power density is very high and can damage the emitting mirror laser facets, leading to slow and long-term corrosion or oxidation of the facets, eventually destroying the device. There is a possibility that it will happen. To prevent either type of damage from occurring, it is necessary to maintain the density of laser power at the facets below a threshold at which such damage occurs. In order to increase the threshold for the above-mentioned damage, a transparent coating may also be formed on the radial facets, for example as shown in US Pat. No. 4,178,564. Although this combination of methods can reduce damage to the laser facet, it is still far from fully utilizing the laser's inherent output power.

It has been found that the fracture of the mirror facets is caused by local heating of the mirror facets to a temperature close to the melting temperature of the material due to the absorption of laser light. To reduce this effect, the semiconductor laser was constructed so that the light-absorbing active layer of the device did not extend into the facets. The region between the active layer and the edges of the facets is made of a light-transmissive material, thereby reducing light absorption problems at the facets. Such devices have been found to have a 5- to 10-fold increase in the threshold power at which long-term catastrophic damage occurs.

However, such devices do not provide lateral mode control, especially in the transparent regions adjacent to the mirror facets. The lateral mode characteristics of the output light beam will therefore depend on the length of the transparent region, which varies from device to device due to the inaccuracy of the cleavage process used to form the facets. Accordingly, it would be desirable to provide a semiconductor laser with lateral mode control that extends to non-absorbing facets.

SUMMARY OF THE INVENTION The present invention relates to a semiconductor laser having a body including a substrate having a pair of parallel mirror facets and a pair of channels substantially parallel to the surface. A first confinement layer is on the surface of the substrate and the channel, and a guide layer with a laterally tapered thickness is on the first confinement layer. The active layer rests on a portion of the guide layer and extends towards the facet from which the light is emitted, but does not contact this facet. The second confinement layer is on the active layer and the confinement region is on the guide layer where no active layer is present. This device has both lateral and transverse mode control extending over the facets. In order to change the shape of the modes in the radial facets, the shape of the guide layer can be varied both in the lateral and transverse directions.

<Description of Embodiments> The present invention will be described in detail below with reference to the drawings.

The semiconductor laser 10 of FIG. 1 consists of a body 12 of single crystal semiconductor material in the form of a parallelepiped. The body 12 has parallel light reflective spaced mirror facets 14a, 14b, at least one of the facets being partially transparent from which light is emitted. The body 12 also has parallel side surfaces 16 spaced perpendicularly between and perpendicular to the facets 14a and 14b.
have.

The semiconductor body 12 has a base body 18, which
8 has parallel first and second surfaces 20 and 22 extending between each facet 14a and 14b and between and perpendicular to the side surfaces 16. A pair of spaced apart, substantially parallel channels 24 are formed within the major surface 20 of the substrate 18;
These channels extend between facets 14a and 14b. Major surface 20 between channels 24
The portion constitutes a mesa (plateau-like portion) 20a. Buffer layer 26 overlies major surface 20 and mesa 20a and partially fills channel 24. A first confinement layer 28 is formed on the buffer layer 26 and a guide layer 30 is formed on the first confinement layer 28. An active layer 32 is formed on a portion of the surface of the guide layer 30 and extends between the side surfaces 16 and extends toward, but not in contact with, the facets 14a and 14b. . A second confinement layer 34 overlies the active layer 32 and a capping layer 36 overlies the second confinement layer 34. The confinement region 38 is on the guide layer 30 and the guide layer 3
This area of 0 is not covered by the active layer 32. Capping layer 36 and confinement region 38
An electrically insulating layer 40 is formed to cover the electrically insulating layer 40, and a through hole 41 is formed in this electrically insulating layer 40. A first electrical contact 42 is formed on electrically insulating layer 40 and capping layer 36 within hole 41 . A second electrical contact 44 is connected to the surface 2 of the substrate 18.
It is formed on 2. Electrical contacts 42 and 4
4 serves as an electrical contact means for the main body 12.

In FIGS. 2-5, reference numbers common to elements in each of these figures are numbered the same as the elements in FIG. FIG. 2 shows a cross section of the laser 10 taken along line 2--2 in FIG. active layer 3
2, the second confinement layer 34 and capping layer 36 are shown extending between the sides 16.

Substrate 18, buffer layer 26, first confinement layer 28
and the guide layer 30 is of one type of conductivity. Second confinement layer 34 and capping layer 40
are of opposite conductivity type. Active layer 32 may be of any conductive type and is generally only lightly conductive. Confinement region 38 may be of any type of conductivity, and is typically of n-type conductivity. The high resistance of confinement region 38 acts to prevent current from flowing around active layer 32 . As another example, confinement region 38 may have a p-n junction and consist of two layers of opposite conductivity type that prevent current from flowing between them.

The refractive index of the active layer 32 at the laser wavelength is greater than the refractive index of the guiding layer 30, which has a refractive index at the laser wavelength that is greater than the refractive index of the guiding layer 30.
is larger than the refractive index of

FIG. 3 shows a facet 14a from which laser light is emitted.
A side view of laser 10 is shown including an overlying optically transparent coating 62. Such a coating is described in U.S. Patent No.
No. 4178564. light reflector 6
4 is formed on the opposite facet 14b.
Effective light reflectors include metals such as those shown in U.S. Pat. No. 3,701,047 and U.S. Pat.
4,092,659.

The various layers are sequentially deposited on base 18 using well-known methods of liquid crystal epitaxy, such as those shown in U.S. Pat. No. 4,215,319 and U.S. Pat. No. 3,753,801. In liquid phase epitaxy, the local growth rate of a layer varies with the local curvature of the surface on which it grows. The greater the amount of local positive curvature of the surface, the higher the local growth rate when viewed from the direction of the superimposed layer. For example, the first confinement layer 28 is grown to such a thickness that the surface of said confinement layer 28, on which the guide layer 30 is deposited, forms a local depression on the mesa 20a. Therefore, the first confinement layer 2 with the maximum positive curvature, i.e. the convexity of the depression
Above part 8, the guide layer 30 has a maximum local growth rate. The upper surface of the guide layer 30 has a convex shape centered on the mesa 20a. Active layer 3 on the guide layer
2's growth rate is higher on Mesa 20 than on Channel 24.
As a result, a tapered active layer 32 is formed with the maximum thickness on the mesa 20a and the thickness decreasing in the lateral direction. Overall, the guide layer 30 has a tapered shape in which the thickness gradually increases in the lateral direction from the portion of the layer above the mesa 20a, while the active layer 28 has a tapered shape in which the thickness gradually decreases in the lateral direction. .

The substrate 18 is preferably made of GaAs.
The main surface 20 is oriented parallel to or preferably deviated from the {100} crystal plane, and has a channel axis aligned parallel to the <110> crystallographic axis. I have it. Using the <110> group of crystallographic axes allows mirror facets 14a and 14b of semiconductor body 12 to
is preferable because it becomes the cleavage plane. Orientation offsets are along the axis of the channel or at an angle with that axis. Preferably, the misorientation of the substrate surface relative to the (001) plane is at an angle between about 5° and 45°, with about 35° being optimal. The inclination angle of the (001) plane from the main surface 20 is approximately
It is between 0.2° and 1.5°, preferably about 1°.
Regarding this, "Semiconductor Laser having High Production Yield"
No. 4,523,318 titled ``Manufacturing Yield''. As another example, the angle of misorientation may be approximately 90° relative to the axis of the channel, as shown in US Pat. No. 4,461,008. When this latter misalignment is adopted, the guide layer has a plateau-like surface, and the active layer has a structure in which the thickness tapers from the portion above the plateau-like surface.

The channel 24 has a dovetail configuration as shown in FIG. 1, so that the channel axis is <110
> parallel to the crystallographic direction. The channel 24 can be, for example, U-shaped, V-shaped, if a different crystallographic axis or a different chemical etchant is used.
Shaped or rectangular shapes can be used. Channel 24 is generally about 4 to 20 microns wide at surface 20, preferably 10 microns, and about 4 microns deep. The groove-to-groove center-to-center distance is generally about 20 to 45 microns, preferably about 32 microns.
It is micron. The grooves are formed using conventional photolithography and etching techniques, as shown in US Pat. No. 4,215,319.

The height of the surface of mesa 20a is
4426701, the height of the major surface 20 above the bottom of the channel 24 is different. This difference in height causes the formation of the mesa to be more curved than if the mesa and its surroundings were of the same height. This height difference is usually about 0.5 to 3 microns, preferably between 1 and 2 microns.

Buffer layer 26 is typically made of the same material as substrate 18 and is typically formed on mesa 20a to a thickness of about 1 to 3 microns. The thickness of this layer varies laterally due to uneven growth rates on the underlying channels and is asymmetric if the substrate surface is misoriented from the {100} plane in a direction nonparallel to the channel axis. become.

The first confinement layer 28 is generally Al _w Ga _1-w Al
The quantity W, which indicates the fractional concentration of Al, is between about 0.25 and about 0.4, and is generally about 0.35. The thickness of this layer is approximately 1 to 3 mm above mesa 20a.
microns, and its thickness varies asymmetrically in the lateral direction.

Guide layer 30 is generally comprised of Al _x Ga _1-x , with a fraction x of Al being less than that of first confinement layer 28 and greater than that of active layer 32, typically from about 0.1 to about 0.3, preferably about 0.2. The guide layer 30 has a tapered shape in which the thickness gradually increases from the part above the mesa 20a.
It has a thickness of about 0.5 to about 0.3 microns above a.

Active layer 32 is generally comprised of Al _y Ga _1-y , with an Al fraction y smaller than that of guide layer 30, typically between about 0 and about 0.07. This layer typically has a thickness of about 0.05 to 0.2 microns on mesa 20a and tapers in thickness in the lateral direction. The active layer 32 typically has a thickness of about 100 mm between the mirror facets 14a and 14b.
extending over a distance of 200 microns,
Approximately 10 to 100 microns of mirror facets, typically approx.
It extends 50 microns inward.

The second confinement layer 34 is generally comprised of Al _z Ga _1-z . The fractionated amount z of Al is approximately 0.3 to 0.5
and preferably about 0.4. Second confinement layer 34 is approximately 1 to 3 microns thick on mesa 20a. Capping layer 36 typically has a thickness of about 0.1 to 0.5
It is micron thick and made of GaAs.

The confinement region 38 is generally comprised of Al _n Ga _1-n , with an Al fraction m between about 0.2 and 0.4, typically about 0.3. Preferably, the thickness of the confinement region 38 is approximately equal to the total thickness of the active layer 32, the second confinement layer 34, and the skipping layer 36, and these layers and the region 38 form a substantially plane plane. .

In constructing the laser of this invention, the active layer, the second confinement layer, and the skipping layer are sequentially deposited over the entire surface of the guide layer using liquid phase epitaxy. The kipping layer, second confinement layer, and portions of the active layer adjacent one or both of the facets are then removed by a series of selective chemical etching steps. Selective etching is preferred because the layer is non-planar due to the channeled substrate. When the capping layer 36 is made of GaAs, for example, 1H ₂ SO ₄ :
It is removed in about 15 minutes at 20°C using a caustic agent of 8H ₂ O ₂ :8H ₂ O or 20H ₂ O ₂ :1NH ₄ OH. When the second confinement layer 34 is made of P-type AlGaAs, 1HF:1H ₂ O or 1HCl:
It is removed using one of the caustic agents 1H ₂ O. The active layer 32 is made of GaAs or Al _q Ga _lq
(where q is less than about 0.07), it is removed using a caustic agent of 20H ₂ O: 1NH ₄ OH. The relative concentrations of these chemical formulas are volumetric concentrations. The concentration q of Al in the active layer is approximately
If the value is less than 0.07, the etchant used to remove this layer will do little to remove the guiding layer 30, if any, underneath. The active layer may be removed by solution etching.

Once the etching process is complete, a mask is formed on the capping layer 36 and a confinement region 38 is grown using standard liquid phase epitaxy. Confinement region 38 may be deposited by vapor phase epitaxy or molecular beam epitaxy. After the deposition process is completed, the mask layer over capping layer 36 is removed and capping layer 36 and confinement region 38 are removed.
An electrically insulating layer 40 is formed thereon.

Electrical insulating layer 40 is preferably comprised of silicon dioxide and is deposited over capping layer 36 and confinement region 38 by pyrolyzing a silicon-containing gas such as silane in oxygen or water vapor.

Using standard photolithography techniques and etching processes, apertures 41 are formed through electrically insulating layer 40 to capping layer 36 on interchannel mesa 20a. It is necessary that the aperture 41 extends to the kipping layer 36.

Electrical contacts 42, preferably comprised of titanium, platinum and gold, are deposited over electrically insulating layer 40 and kipping layer 36 by a series of vacuum depositions. Electrical contacts 44 on second major surface 22 of substrate 18 are formed by vacuum deposition and sintering of tin and gold.

As another example, capping layer 36 and electrically insulating layer 40 may be integrated into a blocking layer of a semiconductor material, such as GaAs, of a conductivity type opposite to that of second confinement layer 34. A portion of the blocking layer, generally in striped form, on the thickest part of the active layer is
It includes an excess concentration of a conductivity type converter that changes the conductivity type of the striped portion to that of the second confinement layer. Reverse biasing the junction between the blocking layer and the second confinement layer by a forward bias voltage applied between the electrical contacts so that a blocking current flows through the layer except in the region of the stripes. .

The effective transverse refractive index variation caused by the thickness and composition of each separate layer can limit the propagation of the laser beam in the lateral direction of the device. In the laser of the present invention, there are two separate regions that must be considered due to their different constructions: the first region, shown in FIGS.
and a second region, also shown in FIGS. 4 and 5, in which no active layer is present on the guide layer. There are two things that contribute to this change in refractive index: the individual volume refractive index of the separate layers and the change in effective refractive index due to differences in the thickness of the individual layers. FIG. 4 schematically shows spatial power variations in the transverse direction in the first and second regions. In the first region, the beam is concentrated at or near the active layer 32, while in the second region the beam is concentrated at or near the active layer 32.
In the region , the beam is concentrated on the guiding layer 30 . In order to maximize the amount of power coupled from the first region to the second region, it is desirable that the effective transverse propagation constants in the two regions join as closely as possible. The concentration of Al in the confinement region 38 is selected such that the effective refractive index in the transverse direction of the second region is approximately equal to that of the first region, and the refractive index in the transverse direction is equal to that of the layer in which the light beam propagates. The above matching can be achieved by combining the respective effects. In this regard, the IEEE Journal of Quantum Electronics
Electronics) Q-17, 78 (1981), Botez's paper.

The output beam of the laser of the present invention can be changed from an elliptical shape of the radiation facet to a substantially circular or other desired shape by changing the shape and taper of the guide layer in the second region. This is done, for example, by varying the height of the mesa between the first and second regions. FIG. 5 schematically shows a cross section of the laser structure in the first and second regions when the height of the mesa above the bottom of the channel in the first region is higher than the height of the mesa in the second region. It is shown in The shape of the guide layer 30 is changed from a taper in which the thickness increases in the first region to a taper in which the thickness decreases in the second region. The beam guided in the second region is thereby more circular than in the first region.

[Brief explanation of drawings]

FIG. 1 is a perspective view of a semiconductor laser of the present invention, FIG. 2 is a cross-sectional view of the laser in FIG. 1 taken along line 2-2,
3 is a side view of the semiconductor laser of FIG. 1 with a facet coating, FIG. 4 is a schematic diagram showing the laser beam intensity distribution in a semiconductor laser according to the invention, and FIG. 5 is a side view of the semiconductor laser of FIG. FIG. 3 is a cross-sectional view of the laser structure in the first and second regions shown and the corresponding laser beam configuration; 10... Semiconductor laser, 12... Main body, 14
a, 14b...Mirror facet, 18...Base, 2
0a... Mesa, 24... Channel, 28... First confinement layer, 30... Guide layer, 32... Active layer, 34... Second confinement layer, 38... Confinement region, 42, 44... ...electrical contact.

Claims (1)

Claims: 1 consisting of a body of material having a pair of opposing mirror facets that are light reflective, at least one of the mirror facets being partially light transmissive; a substrate having first and second opposing major surfaces, the first major surface having a pair of substantially parallel channels extending between the facets, and a mesa between the channels; , a first confinement layer formed on the first main surface of the base and the surfaces of the channel and mesa; and a first confinement layer formed on the first confinement layer, the thickness of which is tapered in the lateral direction. a guide layer; an active layer formed to overlap a portion of the guide layer and extend toward but not in contact with at least one of the mirror facets, the active layer being a part of the guide layer; a first region in the body overlapping the layer and a second region in the body where the active layer does not overlap the guide layer.
a second confinement layer formed overlappingly on the active layer; and a confinement region formed on the guide layer in the second region. a first electrical contact formed on the second confinement layer and a second electrical contact formed on the second major surface of the substrate; The lateral taper of the guiding layer in the region is different, the active layer having a larger refractive index than the guiding layer, and the guiding layer having a larger refractive index than the first and second confinement layers and the confinement region. A semiconductor laser with a refractive index.